1. Field of the Invention
The present invention relates generally to a limited-slip differential gear unit for an automotive vehicle. More particularly, the invention relates to a method and system for adjusting slip limitation in a limited-slip differential gear unit. Further particularly, the invention relates to adjustment of slip-limitation for a limited-slip differential gear unit depending upon a vehicle driving condition.
2. Description of the Background Art
A convention limited-slip differential gear unit has been disclosed in "Jidosha Kogaku Zensho" Vol. No. 9, "Power Train Device", published on Nov. 15, 1980, by Kabushiki Kaisha Sankaido, on pages 321 to 324, for example.
Such a conventional limited-slip differential gear unit employs a slip-limiting mechanism which comprises multi-plate friction clutches. The multi-plate friction clutches are interposed between a differential casing and side gears. For the multi-plate friction clutch, a thrusting force generated by a cam mechanism provided on a pinion mate shaft depending upon driving torque, is applied as a clutch engaging force. According to the clutch engaging force thus generated, a slip-limiting torque is generated by the slip-limiting mechanism. Such types of slip-limiting mechanism have been known as "torque-proportional type slip-limiting mechanisms".
On the other hand, there have been proposed an automotive suspension system which has a variable suspension characteristics such as a damping characteristics. An example of such an adjustable suspension system has been discussed in "Jidosha Kogaku" Vol. 33, No. 10, published on Oct. 1, 1984, by Kabushiki Kaisha Tetsudo Nihonsha, on pages 38 to 55. The disclosed suspension system employs a variable damping force damper which allows adjustment of damping characteristics depending upon vehicle driving conditions or so forth so as to provide both riding comfort and driving stability. In general, softer damping characteristics is selected on city roads for providing better riding comfort. On the other hand, harder damping characteristics is selected while the vehicle is running at high speed so as to provide higher driving stability.
Such conventional limited-slip differential gear units and the adjustable suspension characteristics suspension systems can provide better vehicular driving performance when they are observed independently. For instance, the limited-slip differential gear unit may distribute higher driving torque for higher load wheel by limiting slip at the lower load wheel. On the other hand, the adjustable suspension may achieve both riding comfort and driving stability as set forth above. However, when both the limited-slip differential gear unit and the adjustable suspension system are both applied on a vehicle, problems arise depending on the operation modes of the vehicle. Namely, by adjustment of the suspension characteristics, not only the vehicle body behavior but also load conditions on the wheels may be changed to cause variation of the required driving torque. For example, when a softer suspension mode is selected, limiting of the slip in the differential gear unit tends to become excessive to cause too quick vehicular a response to maintain driving comfort. Furthermore, excessive driving torque on the lighter load wheel may results in degradation of fuel economy. On the other hand, when the softer suspension characteristics is selected, the slip-limiting effect occurs with a certain delay time from the occurence of vehicular rolling. This causes abrupt change of driving characteristics to degrade driveability. Namely, when softer a suspension is selected, though the overall magnitude of vehicle rolling is relatively large, the shifting of the gravity center occurs modelately. Therefore, wheel slip at the inner wheel occurs with a delay time after occurence of vehicular rolling. Since the slip-limitation in the limited-slip differential gear unit becomes effective in response to occurence of the wheel slip, delay time of occurence of the slip-limitation corresponds to delay time of occurence of the wheel slip after vehicular rolling.
On the other hand, when a harder suspension characteristics is selected during high speed crusing, laterial slip of the rear portion of the vehicle tends to occur when slip-limitation is not sufficient.